Vol. 57.] INTRUSIVE TUFF-LIKE ROCKS IN IRELAND. 479 



.29. On Intrusive, Tuff-like, Igneous Kocks and Breccias in 

 . Ireland. By James Eobinson Ktlroe, Esq., and Alexander 

 McHenrt, Esq., M.E,.I.A.^ (Communicated by R. S. Herries, 

 Esq., M.A., Sec.G.S. Read June 19th, 1901.) 



For several years past it has been known to us that fragmental 

 igneous rocks exist in different parts of Ireland, which, though they 

 resemble tuffs, and in certain cases have been described as volcanic 

 rocks, cannot be regarded as ejectamenta, on account of their 

 character and mode of occurrence in the field. Of those which 

 have come more especially under our notice, we may at the outset 

 briefly mention a few particulars, to introduce our subject, before 

 describing in detail the sections exposed in the South-east of Ireland 

 which afford the chief evidence upon which our views of such rocks 

 are based. 



In the Explanatory Memoir (1888) accompanying Sheet 24 of the 

 'Geological Survey Map of Ireland, pp. 34 & 35, certain breccias 

 occurring to the east of Lough Easke, in Donegal, are described as 

 ' agglomerates,' though not in the sense of their having been at any 

 time considered volcanic rocks. In parts, these masses might better 

 be described as crush-breccias, as they, in such cases, follow lines of 

 dislocation. In parts, however, they consist of partly-fused, broken- 

 up, felspathic mica-schist or "• gneiss,' and they merge with felsite- 

 dykes. Sometimes they occur dispersedly in sporadic masses in 

 the mica-schist ; and north-east of Lough Easke the breccia forms 

 a, wide band adjoining the granite, suggesting the conclusion that 

 its formation may be attributed to the earth-stresses which imme- 

 •diately preceded, or in a sense accompanied, the intrusion of the 

 Barnesmore granitic mass. 



Rocks similar to these occur in the district of Forkhill, in Armagh, 

 and are described as ' volcanic agglomerates ' in the Explanatory 

 Memoir (1877) accompanying Sheet 70 of the Geological Survey Map 

 of Ireland, pp. 13, 14, 30, etc. In parts they consist of brecciated 

 slate or brecciated granite and felsite, the fragments being em- 

 bedded in a scanty andesitic matrix. The matrix increases in 

 proportionate quantity downward, and passes by insensible gra- 

 dation into the adjoining felsite, so that no hard-and-fast line of 

 separation can be drawn between this rock and the so-called ' vol- 

 canic agglomerate.' Even where the mass is highly fragmental, the 

 matrix is obviously crystalline, and therefore xenolithic andesite 

 is the term which might more appropriately be applied to the breccia. 

 This fact, considered in conjunction with the gradual passage above 

 mentioned, points to the inference that, as all the fragmental so- 

 caUed volcanic rocks of the region are of the same character, it is 

 doubtful whether clastic rocks of volcanic origin exist there. It is 



^ Communicated by permission of the Director of H.M. Geological Survey. 



